Laser fuses have been used for an extended period of time in the fabrication of integrated circuits. One application of laser fuses is the activation and deactivation of specific functions in an integrated circuit, depending upon intended use of the integrated circuit. For example, a single design for an integrated circuit may be created with a complete set of functionality. However, depending on the price for which the integrated circuit sells, certain functions may be disabled. In another application, the laser fuses permit the replacement of faulty devices and circuits in the integrated circuit with replacement devices and circuits that are operating properly. Typically, when an integrated circuit undergoes testing to verify its operation, faulty portions of the integrated circuit are marked (or stored) by the test equipment. Subsequently, a separate operation is performed wherein certain laser fuses are blown to eliminate the faulty devices and circuits and to replace them with devices and circuits that are operational.
As its name suggests, laser fuses are blown via the use of a high power laser that effectively melts and then vaporizes the fusable links. During the fusing of the fusable links, it is possible for the vaporized fuse material to splatter uncontrollably to adjacent fuses. The splattered fuse material may then cause adjacent fuses to behave incorrectly, i.e., cause a previously blown fuse to behave like an unblown fuse or adjacent blown fuses to become short circuited together. If this happens, the integrated circuit does not behave properly.
Laser fuses come in two major forms, passivated and unpassivated. Passivated laser fuses have a passivation layer formed on top to protect the laser fuse from damage from its operating environment. The use of a passivation layer is especially important for fusable links made from a material that is corrosion-prone, such as copper (Cu). Unpassivated laser fuses do not have the passivation layer and are open to damage from an unfriendly environment. Since unpassivated laser fuses are open to the environment, they tend to be made from corrosion resistant (or relatively corrosion resistant) materials, such as aluminum (Al). Passivated laser fuses have very little sensitivity to the splattering of vaporized fuse material due to the protection afforded by the passivation layer. On the one hand, the passivation layer makes it more difficult to blow the laser fuses. This is due to the fact that the laser used to blow the fuses must have sufficient energy to pass through the passivation layer prior to being able to vaporize the fuse material and to build up sufficient pressure to crack the passivation layer on top of the fusable link to release the vaporized material.
On the other hand, the passivation enables the safe blowing of the laser fuse without affecting neighboring circuits. This is because the passivation layer prevents the immediate and violent release of the molten material of the fusable link. After the fusable link is initially melted by the laser, sufficient energy is absorbed by the fusable link so that the heated fusable link is vaporized. The vaporized material builds up a pressure that will tend to crack its encapsulation material at the material's weakest point, usually the covering passivation layer. The vaporized material explodes from the crack and deposits itself into a very thin and non-conductive film onto the chip surface.
In integrated circuits that are built up by using materials with low mechanical stability, e.g., low-k dielectrics, the cracks created by the release of the vaporized fuse material may appear in the passivation layer and in the underlying dielectric layers. This can cause severe damage to the circuit, particularly if corrosion-sensitive materials such as copper are used for the metal conductive lines. In this case, unpassivated fuses are placed on such integrated circuits to reduce the chance that the underlying surfaces are damaged during the fuse blowing process. To provide an additional measure of protection for the underlying surfaces, there can be a hard dielectric layer placed between the fuse level and the underlying surfaces. Unfortunately, without a passivation layer, the fuse blowing process may suffer in the respect that the molten fuse material may vaporize at a time when there is insufficient heat to prevent the molten metal from being vaporized in its entirety. The vaporization of a portion of the molten metal may result in the splattering of the fuse material that remains in its liquid form. The splattered fuse material may cause electrical short circuits in the unpassivated fuses adjacent to the one being blown. The splattering effect is dependent upon many parameters, such as the power and wavelength of the laser, the dimensions of the fusable link, the material of the fuse material, and the like.
U.S. Pat. No. 6,160,302 proposes the formation of walls between laser fuses to prevent a misaligned laser from unintentionally blowing a fuse that may be adjacent to the fuse that the laser intends to blow.
U.S. Pat. No. 6,300,232 proposes the construction of barriers around individual laser fuses to prevent the propagation of physical damage resulting from the heat induced by the laser during the fuse blowing step.
U.S. Pat. No. 5,899,736 proposes fully enclosing the individual electrically fusable links with a dielectric barrier to prevent the escape of ejected fuse material.
A need has therefore arisen for a way to provide protection for laser fuses that are adjacent to a laser fuse that is being blown without incurring significant cost increases, in terms of additional space requirements and/or additional fabrication steps.